WASHINGTON (CNN) -- Attorney General Michael Mukasey jumped into the political fray in his first week on the job, telling a key Democratic senator he opposed his electronic surveillance plan and would recommend the president veto it if it is passed.

Attorney General Michael Mukasey speaks during his swearing-in ceremony Wednesday in Washington.

In a letter to Senate Judiciary Committee Chairman Patrick Leahy, D-Vermont, on the eve of crucial committee votes to update the Foreign Intelligence Surveillance Act (FISA), Mukasey was adamant in opposing Leahy's plan for changing the law.

Mukasey and Director of National Intelligence Mike McConnell co-signed the letter released Wednesday night by the Justice Department.

"We strongly oppose the proposed substitute amendment. If the substitute is part of a bill that is presented to the president, we and the president's other senior advisers will recommend that he veto the bill," they said.

Leahy last week introduced his substitute to a FISA modernization bill already approved by the Senate Intelligence Committee.

That effort, led by Sen. Jay Rockefeller, D-West Virginia, won wide bipartisan support and is backed by the administration.

It includes retroactive immunity to protect the telecommunications companies that cooperated with the administration's classified no-warrant surveillance program.

Some of the companies are being sued by people who say their privacy rights were violated when phone records, e-mail logs and other information were turned over to the government without court approval.

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Leahy, who opposed Mukasey's confirmation last week, is adamantly opposed to the immunity provision.

Mukasey and McConnell listed almost a dozen other provisions or omissions in the Leahy plan that they said "would not provide the intelligence community with the tools it needs effectively to collect foreign intelligence information vital for the security of the nation."

Leahy and many of his Democratic allies back provisions they believe provide essential civil liberties protections against excessive government intrusion and potential abuse.

The veto threat adds to an already testy atmosphere in which the highly partisan Senate Judiciary Committee has struggled to reach a consensus on how to update the 30-year-old FISA law, which they agree has been overtaken by dramatic changes in technology.E-mail to a friend